The present invention relates to an electrolyte membrane including a graft polymer having a sulfonic acid group as a proton conductive group, in which, when the electrolyte membrane is divided into four equal parts in a thickness direction thereof, a content of the sulfonic acid group in each of outer regions is larger than a content of the sulfonic acid group in each of inner regions; in which A1, A2, B1 and B2 satisfy the following formula: 1.5≦(A1+A2)/(B1+B2)≦8, in which A1 and A2 each represent a maximum value of a distribution amount of the sulfonic acid group in each of the two outer regions, and B1 and B2 each represent an average value of a maximum value and a minimum value of a distribution amount of the sulfonic acid group in each of the two inner regions; and in which the electrolyte membrane has an ion-exchange capacity of 0.5 to 2 meq/g.